1. Field of the Invention
This invention relates to facsimile (fax) devices and more particularly to fax adapter devices and methods of operation.
2. Description of the Related Art
Certain conventional fax machines transmit and receive non-secured data using group 3 (G3) fax protocol over a Public Switched Telephone Network (PSTN). Certain other fax machines transmit and receive secured data over a secured network using a secure protocol such as Secured Telephone Unit (STU) protocol. In recent years, the cost of G3 fax machines has decreased and conventional G3 fax machines have proliferated. These conventional G3 fax machines however, cannot interface directly with an STU. In certain instances, it is desirable to couple a conventional G3 fax to a secured network via an STU. In these instances, secure fax adapters are used for coupling a conventional G3 fax to an STU.
Secure fax adapters conventionally use a Data Access Arrangement (DAA) to perform several Telephone Central Office (TCO) functions including off-hook detection and ring generation. Conventional DAA off-hook detection circuits measure a voltage drop across a load to arrive at an on- or off-hook decision. Conventional off-hook detection circuits do not adequately current limit to protect defective or incompatible devices coupled to the secure fax adapter. Additionally, some conventional off-hook detection circuits have a diode in the sensing signal path which has the undesirable effect of introducing distortion to the sensing signal.
Ring generation is another TCO function typically performed by a secure fax adapter DAA. The DAA of some conventional adapters use a transformer to generate a sinusoid for ring generation. Use of a transformer, however, is undesirable due to the added cost, weight and size.
Thus, there is a need for a low cost, light weight and compact secured fax adapter having an enhanced DAA with improved ring generation and off-hook detection and providing increased flexibility in operating configuration.